The present invention relates generally to display systems, and more particularly to a miniature display system in which a ferroelectric liquid crystal (FLC) spatial light modulator is positioned directly on top of and controlled by a specifically written (i.e. addressed) very-large-scale-integration (VLSI) silicon integrated circuit (IC) backplane.
In the field of miniature displays and especially those using spatial light modulators, it is well known that stationary and moving images, either monochrome or color, may be sampled and both color separated and gray scale separated. These separations may be digitized forming digitized images which correspond to the given images. These digitized images are used by devices in this field to create visual images that can be used for a direct visual display, a projected display, a printer device, or for driving other devices that use visual images as their input. The present invention provides a novel spatial light modulator using novel data sequencing schemes which may be used to produce a direct view miniature display or a projected display, or for driving a printer or other devices that use visual images as their input.
One of the ongoing challenges facing the manufacture of miniature display systems is providing smaller and smaller displays with higher and higher resolution. The combination of these two factors dictates smaller and smaller pixel sizes in order to achieve smaller displays with higher resolution. Miniature displays which are small enough to be mounted onto a helmet or displays small enough to be supported by a pair of eyeglasses will find a wide variety of uses if they can provide adequate resolution and brightness in a small, low-power package at a low cost. Conventional display technologies such as CRTs are difficult to miniaturize and therefore do not hold much promise in this field. Alternatively, new displays based on VLSI integrated circuits are currently being developed. Examples of these miniature displays in the prior art include digital micromirror devices, active-matrix electroluminescent displays, and active-matrix liquid crystal displays. However, each of these prior art devices has significant limitations in resolution, brightness, compactness, lumination efficiency, or color capability. Furthermore, because of various physical restrictions on systems of these types, the smallest pixel pitch that to applicant""s knowledge has been achievable thus far has been a pitch much greater than 10 microns. As will be seen hereinafter, the present invention provides a display capable of providing smaller pixels, for example with a pitch on the order of 10 microns or less, with higher resolution in a bright, full color capable display. As will also be seen, the present invention provides for unique data ordering schemes, and a unique method of illuminating the display, as well as other features which will become apparent.
As will be described in more detail hereinafter, a system for producing modulated monochrome or color light having gray scale is disclosed. The system includes an active matrix liquid crystal spatial light modulator having light modulating means including (i) a layer of ferroelectric liquid crystal material which is designed to switch between ON and OFF states and (ii) active matrix means including VLSI circuitry for dividing the layer of liquid crystal material into an array of individual liquid crystal pixels and for causing each of the pixels of liquid crystal material to modulate light individually by switching between the ON and OFF states in a way that depends upon the data with which the VLSI circuitry is written. The system also includes illumination means having a light source for directing light from the source into the pixel-divided layer of ferroelectric liquid crystal material in, a specific way. And finally, the system includes means for writing the VLSI circuitry with preselcted data in accordance with a particular data ordering scheme, such that the circuitry, in response to the data, causes the pixels of liquid crystal material to individually switch between their ON and OFF states and therefore modulate light from the source in a way which, depending upon the data, produces a specific overall pattern of gray scale light.
In accordance with one feature of the present invention, simplified VLSI circuitry, specifically circuitry using a single active component per pixel, is combined with a very thin layer of ferroelectric liquid crystal material, approximately one micron thick, to produce a spatial light modulator for a display in which the spatial light modulator is comprised of extremely small pixels. This feature of the present inventions allows a spatial light modulator with a pixel pitch of 10 microns or less to be produced. In a specific embodiment disclosed herein, the spatial light modulator includes a thin layer of ferroelectric liquid crystal confined between a silicon VLSI circuitry backplane and a glass window coated on its inner side with a transparent electrode layer of indium-tin oxide. The VLSI backplane includes an array of conductive metal pads positioned on the upper surface of the VLSI backplane. The conductive metal pads each have a reflective top surface which is designed to reflect light directed into the spatial light modulator back out of the spatial light modulator and also act as electrodes controlled by the data writing means. The conductive metal pad electrodes and the transparent electrode layer are positioned on opposite sides of the ferroelectric liquid crystal layer and are used to form electric fields through the layer of individually controllable ferroelectric liquid crystal pixels whose positions correspond to the positions of the conductive metal pad electrodes.
In accordance with another feature, different very specifically configured data ordering schemes compatible with the simplified VLSI circuitry are utilized in order to achieve controllable gray scale. Each of these data ordering schemes contemplates first establishing a particular frame rate for the display system. Each of these frames is then divided into a plurality of subframes for purposes of obtaining monochromatic gray scale in the case of a monochromatic display or subframes for purposes of obtaining color and sub-subframes for obtaining color gray scale in the case of a color display.
In accordance with one data ordering scheme, the gray scale of any given pixel is obtained by dividing each frame into subframes of time periods, preferably time periods of unequal length, while maintaining the light directed into the modulator at a fixed brightness and by either turning ON or OFF the particular pixel during certain subframes of the frame such that the cumulative time in which the pixel is ON during the frame is proportional to the desired gray scale for that pixel. Alternatively, in accordance with a second data ordering scheme, the gray scale of any given pixel is obtained by dividing each frame into subframes of equal time periods and by turning ON or OFF the particular pixel during certain subframes of the frame while at the same time changing the brightness of light directed into the spatial light modulator for the different subframes. The cumulative time in which the pixel is ON, in combination with the brightness of the light during each subframe, is proportional to the desired gray scale for that pixel. A third scheme is a combination of the first scheme and the second scheme. In each of these schemes, because of the frame rate, the viewer""s eye perceives the time integral of brightness of each pixel for each frame, thereby perceiving the desired gray scale image. In both of these second and third data ordering schemes, additional blackout subframes are provided in accordance with the present invention in order to maintain the fidelity of the ultimately produced display.
In accordance with another feature of the present invention, the three different data ordering schemes are utilized in order to achieve a color display with gray scale for each color. This is done by establishing a particular frame rate for the display system and dividing the frames into a plurality of subframes for purposes of obtaining color. During each of these subframnes light of a certain color is directed into the spatial light modulator. The subframes are further divided into a plurality of sub-subframes for purposes of obtaining gray scale for each color subframe, as indicated above. In all three of these color schemes, blackout subframes and/or blackout sub-subframes may be used. In accordance with still another feature of the present invention, individual light emitting diodes are used as color light sources for a color display.
In accordance with a more general embodiment of the present invention, a system for producing modulated light is disclosed including a spatial light modulator having an array of individual light modulating pixels. Each pixel includes a light modulating medium which is designed to switch between ON and OFF light modulating states for modulating light. depending upon the pixels light modulating state, and means for switching the pixels associated light modulating medium between the medium""s ON and OFF states in response to and dependent on externally generated, preselected data signals. In one example of this embodiment, the modulating medium is ferroelectric liquid crystal which is switched using a digital approach from between only a fully ON and a fully OFF state. In a second example of this embodiment the modulating medium is nematic liquid crystal which is controllably switched using an analog approach to a fully ON state, a fully OFF state, or any partially ON state. The system also includes means for generating a stream of the preselected data signals and means for applying the preselected data signals to the switching means associated with each of the pixels during each one of a series of successive time periods such that (i) for any given successive time period, all of the pixels are first caused to modulate light depending upon the states of their respective light modulating mediums and responsive to the data signals applied to the light modulating mediums during the given period, and (ii) thereafter during certain ones of the time periods, the entire array of pixels are switched to the OFF state, all at the same time, whereby to provide a pixel blackout subperiod during each of the certain time periods.